Electrochemical devices, such as batteries, are widely used in portable and auxiliary power supplies. The basic working unit of a battery is an electrochemical cell. The electrochemical cell includes two electrodes (an anode and a cathode) and an electrolyte. The battery electrolyte may be a liquid, solid, or gel. The electrolyte provides a path for ions to flow from the cathode to the anode (charging) as well as for the ions to flow from the anode to the cathode (discharging). The battery will not work if the cathode and anode make electrical contact.
A separator is used to “separate” the cathode from the anode, serving as an electrical barrier between the cathode and the anode. Although the separator is an electrical barrier, the separator may not be an ionic barrier. In some instances, to maximize ionic flow, the separator is as thin and as porous as possible. A separator may be a thin porous polymer film.
Void spaces in the separator polymer are filled with electrolyte that also fills pores in the anode and cathode coatings. An organic alkyl carbonate containing selected lithium salts is one example of a liquid electrolyte. The electrolytes offer a high mobility of ions (e.g., lithium ions) and are designed to be chemically inert when exposed to the voltage potential at the cathode and anode surfaces.
Due to its electrical storage capacity, the lithium secondary (rechargeable) battery has become a preferred electrical storage device for hybrid and electric vehicles, electric grid storage, and a multitude of portable consumer electronics such as laptop computers, cellphones, and hand tools. The higher storage capacity comes from a combination of higher voltage potential and greater energy density (ion density) within the electrode surfaces.
With higher voltages and energy density comes greater risk of fire. The separator is a key component to preventing fire. Fire can occur if 1) the battery discharges so quickly that the corresponding heat melts or shrinks the separator, 2) physical damage to the battery causes the anode and cathode to touch, or 3) electrolytic plating (irreversible side reactions) cause lithium ions to plate lithium metal on the anode in such a way that over time they develop lithium growths (e.g., dendrites, spikes, etc.) on the anode that keep growing until they form a metallic bridge to the cathode.
Example separator films include thermoplastic polypropylene (PP), polyethylene (PE), or coextruded blends of PE and PP. One of the advantages of the PE or PP separator is that these thermoplastic polymers flow when exposed to heat. This heat induced flow causes the pores in the separator to close. When the pores close, the separator is a barrier to ionic flow. So in cases of mild or gradual overheating states, the thermoplastic separator shuts the battery down.
Thermoplastic PE-PP, however, have several disadvantages. Thermoplastic PE-PP separators are very similar in strength and heat resistance to that of a common kitchen sandwich bag. In the event of battery rupture, PE-PP separators provide insignificant mechanical strength; and in the event of fast discharge, PE-PP separators do not have the heat resistance to remain in place. In high heat conditions, the polymer separator can go from melting, to curling, depolymerization, and decomposition. As the polymer separator film curls or decomposes, the barrier between the cathode and anode vanishes. In this state, fire will break out if the battery cannot be shut down immediately.
In view of fire safety considerations, a superior, porous, mechanically strong, heat resistant, and stable separator is desired, wherein the separator does not form cracks or cause short circuits due to shrinkage when the electrochemical cell is either heated or compressed.